Micro-electromechanical systems (MEMS) have been extensively used as sensors for a variety of mechanical parameters such as pressure, flow, mass, and stress. In addition, they have been used for several electrical applications, such as filters and switches in Integrated Circuits and high quality factor (high-Q) oscillators. Micro-electromechanical systems (MEMS) and more recently nano-electromechanical systems (NEMS) have also been widely studied for the detection of chemicals and bio-molecules. Chemical sensors have widespread industrial and environmental applications including toxic gas sensing, process flow monitoring, and pollution control. Bio-molecule sensors on the other hand are extremely useful for label-free detection in lab-on-a-chip type applications, bio-hazard detection for counter-terrorism, and homeland security.
The microcantilever used in a scanning probe microscope (SPM) in non-contact or tapping modes is essentially a MEMS device. The advantage of using a microcantilever in measurements such as in a SPM, include high quality factors of such oscillatory systems, easy batch fabrication using standard lithographic techniques, and a base material (commonly Si) with tunable electrical properties. In recent years, there has been a large focus on using the micro(nano)cantilevers as sensor elements for specific chemical and biological species. These applications exploit the above mentioned advantages associated with micro(nano)cantilevers, in addition to the requirements of low power and very small sample quantity for detection.
In a MEMS/NEMS detection system, an electrical signal is converted to mechanical oscillations of a resonator, which under the influence of external agents (analyte molecules), changes its resonance characteristics. This change is then converted back into an electrical signal enabling detection. MEMS based sensing is usually preferred due to its extremely high sensitivity resulting from its very high quality factor (in the range of 105-107 in vacuum), which greatly exceeds that of its electrical counterparts. In addition, the power consumed by these sensors is very low, so a large array of these sensors can be packed together with very low power dissipation, for lab-on-a-chip or multi-functional sensing applications. There are two basic mechanisms by which the MEMS based devices commonly sense analyte molecules: (i) change in resonance curve due to a change in mass attached to the cantilever or beam resonator, and (ii) change in resonance curve due to the stress induced in the cantilever or beam by the attached molecules.
In the past, one of the most common techniques to detect specific types of molecules (usually larger bio-molecules) is to detect the change in resonance frequency of very high-Q cantilevers due to specific molecular attachment (causing changes in mass). The specificity is obtained through functionalization of the cantilever surface using a specific coating that enables the attachment of target molecules. Using micro(nano)cantilever resonators in high vacuum conditions, mass detection down to sub-attogram level has been demonstrated. Another very commonly used detection methodology involves using a chemical field effect transistor (Chem-FET), which is basically a gateless FET whose surface is functionalized with appropriate coating for specific molecular attachment. In such a device, the surface barrier potential changes due to attachment of the target molecules. Since the barrier potential change is related to the current, the latter can be measured to detect the attachment of the target molecules. In these past detection systems, the external agents (analyte molecules) attach themselves directly to the functionalized surface of the microcantilever. Thus, after exposing the microcantilever to an environment to be tested, the surface of the microcantilever must be cleaned (i.e., remove any attached analytes from the surface) or replaced. Presently, it is more economical to simply replace the microcantilever after each use. However, replacing the microcantilever after each use increases the cost dramatically, not to mention being wasteful.
As such, a need exists to allow for a method and system that employs a reusable microcantilever for the detection of analytes.